In conventional building construction, door frames are constructed on site with the wooden jambs, threshold and lintel nailed together in place and attached to the studs framing the door opening. The trim members are then nailed in place about the door frame prior to the application of the wall sheathing.
More recently, factory constructed and prehung door frames have been used which can be shipped as a unit to the building site. Door frames constructed in this manner can, of course, be constructed to rather specific tolerances. However, the door frames must be attached to the studding framing the door opening, and the studs are usually not installed with the same degree of accuracy that the prehung door frames are manufactured so that some difficulty may be experienced in mounting a preconstructed door frame in the door opening.
Conventionally, the door trim or brick molding is also constructed on site and the side trim members and the head trim mitered and nailed in place around the door opening. This, of course, is time consuming and requires relatively expensive skilled labor for both constructing and installing the trim.